The present invention relates to a method and an apparatus for plastically forming helical internal gears and helical gears, and more particularly to a plastically forming method and apparatus for extruding internal helical gears and helical gears by pushing materials processed to any type of blank into a die unit successively by means of a punch, i.e., by passing the materials once through the die unit.
To date, there have been known several apparatuses for plastically extruding helical gears which have helical teeth, as disclosed in U.S. Pat. No. 3,605,475 and No. 3,910,091 by way of example.
Such a helical gear extruding apparatus comprises a combination of a die having a helical gear toothed section formed on its inner wall surface, a container integral with the die, a mandrel disposed in alignment with the axes of the die and the container, and a punch for pushing metal material blanks into the container and the die successively to thereby extrude helical gears.
In the helical gear extruding apparatus as mentioned above, while the mandrel and the die are circumferentially rotatable relative to each other, the die is integral with the container, and the metal material blank being pushed is not circumferentially rotatable relative to the die. Therefore, when the metal material is pushed into the die to form helical teeth on the outer peripheral surface of the metal material, the material is subjected to axial flow (extension), which acts to form the product toothed portion with a smaller helical angle than that of the die toothed portion and hence produces a lead gap between the die toothed portion and the material toothed portion under molding. From this there may arise a problem. Specifically, great stress is produced on the surfaces of the respective teeth of the die and the metal material on one side, causing a pressure difference between the left-hand and right-hand sides of the molded toothed portion, including elastic recovery, with respect to the die toothed portion. This may cause the molded toothed portion to seize or bite the die toothed portion. In the worst case, the die toothed portion would be damaged.
Further, in order to prevent axial extension of the metal material during extrusion, the above-cited U.S. Pat. No. 3,605,475 adopts a technique to make the hollow portion of the metal material free from constraint by omitting the mandrel, and hence allow flow of the material toward the inner peripheral side thereof.
While this technique is effective in reducing the lead gap, it gives rise to a problem that high-accuracy helical teeth cannot be obtained because of reduction in the three-dimensional constraint force acting from the inner and outer peripheral surfaces of the material and in the axial direction thereof during flow deformation. Another problem is that accuracy of the inner diameter of the helical gear is reduced as well.
At present, therefore, although several techniques for plastically forming helical gears have been proposed as disclosed in the above-cited U.S. Patents, the technology capable of mass-producing helical gears on an industrial basis has not yet been established. Thus, notwithstanding the fact that helical gears are principal components suitable for transmitting rotation in many machines, including transmissions for automobiles and motorcycles, they are currently formed by cutting with gear hobbing machines. In addition, no methods of plastically formed helical terminal gears have been reported in Japan or elsewhere in the world. As with the above case, notwithstanding the fact that internal helical gears are principal components suitable for transmitting rotation in many machines, including transmissions for automobiles and motorcycles, they are currently formed by cutting with broaching machines.